Most commercially proven solar air heating systems are transpired air panels (i.e. air flows through the surface of the panel) installed on exterior building walls, to preheat fresh ventilation air for the building heating system. For example, U.S. Pat. No. 4,774,932 to Hollick describes a flat or vertically corrugated panel covered with perforations, such as holes or slits, and spaced off the south facing building wall. Solar radiation heats up the absorbent face of the panel and the building heating system fans pull in the solar-heated boundary layer air that collects on the outside surface of the panel, through the perforations into the plenum behind the panel, and then into the heating system intake. This is an engineered system for a large panel area and high ventilation air flows.
Ambient wind is a significant factor that reduces the efficiency of this system. If the transpired air flow through the perforations is less than about 2 cubic feet per square foot of panel, the heated boundary layer air collects on the panel surface and may be blown away by wind. This is a limiting factor in scaling the system down to small commercial and residential applications, wherein small panel areas and low solar heated air flows are required. Another efficiency reducing factor arises from the fact that the panel is normally mounted on a vertical building wall. This 90° angle to the ground is not perpendicular to the daily and seasonal sun's path angle (averages about the degree of latitude of the installation). Thus some sun radiation is reflected off the panel and represents a 20% to 25% loss in radiation on the panel, compared to a panel that is perpendicular to the sun angle over the daily sun path.
The Hollick transpired air solar heating system has been implemented as a commercially successful solar air heater, primarily for building ventilation and heating systems. The only common solar heating systems in residential and small commercial markets are solar hot water heaters. Solar hot water heating systems also have several problems. The installation cost is high as professional installation is required, and even in high utility energy cost locations and with incentives, the economic payback for the system, based on energy cost savings, takes several years. This factor also applies to solar air heating systems that have been tried in residential markets.
Current unglazed transpired solar (UTS) air heaters are disclosed in U.S. Pat. No. 4,744,932 to Hollick, U.S. Pat. No. 5,692,491 to Christiansen et al, and U.S. Pat. No. 7,677,243 to McClenden, and use the suction side of a ventilation fan to draw in ambient fresh air through the transpired air panel perforations in the thermal absorbent surface. This also draws in solar-heated boundary layer air collecting on the solar radiation surface of the panel. The fresh air and heated boundary layer air are mixed in the plenum behind the transpired air panel. This is a single pass system and the heating capacity is affected by ambient wind that will blow heated boundary layer air off the UTS panel.
Glazed transpired solar (GTS) heaters, as disclosed in U.S. Pat. No. 4,323,054 to Hummel, U.S. Pat. No. 7,434,577 to Doherty, U.S. Pat. No. 4,159,707 to Mique, U.S. Pat. No. 4,034,736 to Telkes, U.S. Pat. No. 6,109,258 to Rylewski, and U.S. Pat. No. 4,090,494 to Borst et al., operate in a similar method as described above but are more efficient, even with the loss of radiation transmission through the glaze, because the glaze prevents ambient wind from blowing the heated boundary layer air off the transpired panel surface as happens on non-glazed UTS panels. These are also single pass systems.
The porosity of UTS transpired air panels is also a critical factor in the thermal performance of the system. The Department of Mechanical Engineering, University of Auckland, New Zealand study entitled Use of Perforated Metal Sheets as Solar Collectors for Building Space Heating, August 2004 shows that the optimum collector porosity was 0.2% to 0.4% of the total surface area of the panel. Northern hemisphere panels use porosities in the range of 1.0% to 2.0%. It was also recognized that reduced fan suction rates and high wind speeds also affected UTS collector efficiency.
U.S. Pat. No. 7,434,577 to Doherty discloses a series of fixed orientation angle flat solar heated collectors that heat the air flow in stages. Each collector front edge is in contact with a front glaze panel and the back edge is in contact with the back panel. The air flow enters the edge of the collector into a series of partitions inside each collector parallel to the length of the collector and is heated by the thermal conductivity of the collector. There is no direct air flow contact with heated boundary layer air on the surface of each collector other than from convection into the air flow to the next collector inlet. Staged heated air also accumulates under the upper bottom surface of the next angled collector where it is in contact with the back panel and out of the air flow path. While this adds bottom thermal conduction heat to the air flow inside the collector, there is limited mixing of air thermal transmission.
U.S. Pat. No. 4,159,707 to Miguel, U.S. Pat. No. 4,034,736 to Telkes, U.S. Pat. No. 6,109,258 to Rylewski and U.S. Pat. No. 4,090,494 to Borst et al. disclose staged adjustable or fixed collectors. Rylewski discloses transparent collectors with a glaze and back panel in contact with each collector, but no fresh or recirculating airflow. Telkes and Miguel disclose staged collectors but the air flow can move past each stage between the glaze and the collector. Borst discloses a fixed collector plate formed into perforated louvers. Solar radiation only falls on the tilted front perforated surface to draw solar heated boundary layer in to the air flow like Hollick. These solar air heaters are also of a rigid construction.
The successful solar air or water heating product for residential and small commercial markets should be manufacturable in high volume; flexible and modular to suit the variety of applications and installations in these markets; simple and easy to install; easy to ship and store; attractive to suit residential applications; and have a high solar heating efficiency and a short economic return.